Ion Mobility spectrometers (IMS) have been developed for very sensitive monitoring of chemical species. An important application of the technology is its use to detect trace quantities of explosive material and chemical agents in compact devices. Trace ions of a subject material are separated by using the fact that the electrical mobility of different ion species is different. Time of flight measurements are made. Once chemical species are separated, the ionic current is measured using an electrometer.
Presently, most IMS devices use for ionizing a sample either a radioactive source (Ni63 being the most common, although T3 and Am241 are also used), UV or corona discharge. Use of more intense ionization sources could be advantageous, since the current measured by the electrometer would be larger, decreasing the detection threshold. If limited by ion recombination, the density of reactive ions (and therefore the current) is proportional to the square root of the ionization source. Increasing the ionization rate by a factor of one-thousand increases concentration of reactive ions (primary ions) by a factor of approximately thirty, which should result in a decrease in the minimum detectable limit (MDL) by a factor of thirty or higher. Moreover, there are cases (where the ions quickly form cluster ions, for example) where the ion loss is not determined by ion recombination. In this case, the ion concentration will be linear with ionization rate, and the ion concentration will be one-thousand times higher for an ionization source one-thousand times stronger.
Traditionally, the sample analyzed by IMS is on the order of a few hundred ml/min. This small sample size is partly due to the very low dose of the ionization source used in these instruments. Increasing the sample size by an order of magnitude can increase the accuracy and the sensitivity of the IMS. This increase can be achieved with the use of a more powerful ionization source such as an electron beam.
The use of more intense ionization sources for IMS has been previously considered by others. Electron beams have also been contemplated as an ionization source. In the patent literature, Vitaly Budovich (U.S. Pat. No. 5,969,349, October 1999) teaches the use of an electron beam as the ionization source. The source has a window, preferably mica, and an evacuated volume with a hot cathode or a photocathode. Hans-Rudiger Donzig (U.S. Pat. No. 6,429,426, August 2002) teaches the use of an electron beam source used to make x-rays. In this case, the electrons do not have to be extracted from the evacuated volume. More recently, Hans-Rudiger Donzig (U.S. Pat. No. 6,586,729, July 2003), teaches the use of a current controlled e-beam for the control of x-ray emission, using a sustainer (in a triode configuration). This patent also teaches a scheme for monitoring the pressure in the tube and evacuating the tube when the pressure is too high.
These patents, and in particular U.S. Pat. No. 5,969,349, teach an electron source with a cathode at a high negative potential. This high potential is needed for acceleration of the electrons using conventional acceleration technology. However, conventional technologies present issues with the size of DC power supplies (including the transformer), the size of the high voltage insulators and other issues dealing with high voltage such as arcing. Alternatives to the high voltage requirement for high energy electron beams could result in significantly more robust and compact devices.
In U.S. Pat. Nos. 5,969,349, 6,429,426 and 6,586,729, no mention is made of the possibility of using a variable strength ionization source for the optimal performance of the IMS, nor do they teach operation of IMS to handle the large space charge that is generated by a source that is much stronger than the conventional radioactive sources in IMS devices. Large space charge is not an issue with U.S. Pat. Nos. 6,429,426 and 6,586,729, due to the very low efficiency in turning electron energy into soft x-rays.
It is very important, while increasing the intensity of the ionization source, to decrease the effect of space charge in the drift region in order to take full advantage of the higher ion concentration. High space charge at the higher ion concentration limits the resolution by spreading the peaks and by ion radial loss in the drift column due to space charge.
An approach that uses nonlinear effects on mobility for chemical species separation has been proposed. This approach employs High Field Asymmetric Ion Mobility Spectrometry (FAIMS) (I. A. Buryakov, E. V. Krylov, E. G. Nazarov, U. K. Rasulev, Int. J. Mass Spectrom. Ion Processes 128 (1993) 143; R. W. Purves, R. Guevremont, S. Day, C. W. Pipich, M. S. Matyjaszczyk, Rev. Sci. Instrum. 69 (1998) 4094.)). See also, U.S. Pat. No. 5,420,424, ION MOBILITY SPECTROMETER, BL Carnahan, A. Tarassov Apr. 29, 1995. In this case, the ion separation occurs by applying a combination of DC and AC fields in the direction perpendicular to the motion of the sample gas flow. The ions are separated due to nonlinearity of the ion speed with respect to the applied electric field. A combination of DC and AC fields results in no net drift for a specific set of ions, which after separation can thus be injected into a mass spectrometer (MS). The applied electric fields are perpendicular to the direction of gas flow, and the ions are separated/removed in the direction perpendicular to the gas flow. The advantage of this scheme is that it is possible to have continuous injection into the MS (as opposed to a regular IMS that has pulsed injection of the ions of interest, known, and the product ions). However, the separation in this approach (FAIMS/MS) occurs in the drift region, and does not solve the problem of high space charge.
Alternative methods for concentrating the ions were discussed by William Blanchard (U.S. Pat. No. 4,855,595, August 1989). This patent teaches the concentration of the sample ions using electric fields in the drift region, downstream from the shutter. The issue of high space charge in the region upstream from the shutter is not addressed, nor is any preconcentration or space charge reduction upstream from the shutter. The problem is not resolved by concentrating in the drift region, since the largest space charge occurs immediately after the shutter grid, before the ions have had time to axially separate.
A model for the sheath region has been developed. The sheath region is defined as the region where species of either positive or negative charge exist, surrounding a region with very similar positive and negative ion concentrations. Results are shown in FIGS. 1a and 1b. For the case of 2 nA (FIG. 1a) (typical of present day devices with a current density on the order of 10−5 A/m2) the dimension of the space charge region (known as sheath) upstream from the shutter can be on the order of a few centimeters (IMS design used for the calculations was obtained from Analysis of a drift tube at ambient pressure: Models and precise measurements in ion mobility spectrometry, G. A. Eiceman, E. G. Nazarov, and J. E. Rodriguez, J. A. Stone, Review of Scientific Instruments 72 3610 (2001)). The distance between the shutter and the ionization region in conventional IMS systems is comparable to the sheath dimension. For this reason, these systems are not strongly affected by space charge in the ionization region. The electric field is strongest in the zone next to the shutter grid, and approaches zero at the location of the plasma zone.
If the ionization strength is increased, the sheath size is reduced. Results for the case of current density on the order of 200 nA are shown in FIG. 1b. In this case, the size of the sheath is less than 1 cm. Space charge is important in this case, and care must be taken in the latter case with the ion injection method to minimize space charge in the drift region. Conventional injection methods would result in space-charge dominated flow in the drift region, with loss of resolution and corresponding increase in Minimum Detection Level.
Present day devices utilize a relatively low radioactive source, about 10 mC (milliCuries). This ionization source produces high energy electrons with a current of about 15 pA (picoAmperes). It should be noted that these fast electrons ionize the background gas producing a swarm of electron/ion pairs, at an energy expense of ˜35 eV per electron/ion pair. With little difficulty it is possible to have an electron beam with currents of a few μA (microA), while with difficulty it is possible to have electron currents on the order of mA (milliA). This results in a very large increase in the ionization source, about 5 orders of magnitude for the case with a 1 μA (microA) beam. It should be noted that alternative ionization sources in otherwise conventional IMS devices (such as corona discharge) operate at currents on the order of a μA (microA).
In the drift region, the space charge limits the resolution of the instrument. With 2 nA current in the drift region, present day IMS have substantial space charge to result in substantial spreading of the ion cloud (and therefore loss of resolution and selectivity). With a 100 μs pulse width (corresponding to an axial cloud length of the about 1 mm), a 2 nA beam will spread about 50 μs (corresponding to about 0.5 mm axial length of the cloud). Therefore, space charge in the drift region is already important in present day devices, and needs to be addressed for devices with much higher ionization rate for improved resolution in present day devices.
Diffusion of the ions is another source of broadening. It has been known that the resolution (selectivity) limitation due to diffusion depends on the voltage applied across the drift region. For the highest voltages considered, diffusion is less important, and the spreading is the combined effect of both space charge and diffusion.
Although a large amount of literature exists on IMS devices, little is said about ion handling upstream from the shutter. In this region, high space charge results in plasma surrounded by sheaths. Improved performance of the IMS can be obtained by using innovative methods of ion injection into the drift region, including shutter grid design.
FIG. 2a shows the potential distortion, due to finite geometry effects, in the shutter region for those conditions with the shutter closed. FIG. 2b shows the electric field with the shutter open (data from Eiceman et al.). The ion reaction region is to the left of the shutter, while the drift region is to the right. The grid spacing and wire size were taken from Eiceman (0.05 mm parallel wires separated 0.5 mm). Spikes occur at the location of the parallel wires. Note the large distortion of the potential due to shutter grid geometry, and in particular, that the potential to repel the positive ions is substantially less than the applied potential. It is during transients (i.e., when the shutter is open) that the distortion effects are important in accepting ions. This is important because the ions are extracted mainly from the regions with large field distortion (the width of the cloud is about 1 mm, and the region of highly distorted fields is about 0.5 mm).